I try not to keep stuff that will need to be stored for more than a couple of years. I can be a bit of a hoarder and tend to feel like it will save money and have sentimental value, but really, my kids have Brio that has been through my cousin, my half siblings and all three of my children (two of whom have a 10 year age gap). None of it was played with by my sister or I when we were little because our Brio was a totally different set, but it's still nostalgic and familiar to play with the Brio that we have. So when they lose interest in it, I will pass it back to my aunt and if I have grandchildren one day, I'll pick up a job lot of secondhand wooden train track.
Some plastic toys can get brittle and stop working when they are stored. Really old electrical items never seem to work either. We were passed down a Fisher Price electric, singing, teething toy that by my reckoning is about 12-15 years old, my 1yo was playing it one day and I realised that the rubbery part was all crumbling into pieces causing a serious choking hazard. And if you have old vintage plastic toys from the 80s or earlier, you should really check whether that company was using heavy metals in its plastics before guidelines were tightened up in the 90s and 00s. Many of them are no longer advised to play with.
My aunt kept clothes from my cousin and tried to pass them to my stepmother for my half siblings and they were 15/20 years old by this point and my stepmum privately said that while it was kind, they all looked naff because the style of children's clothes had changed. We don't tend to think of children's clothing as following fashion, but it does to an extent and something from 15+ years ago will look noticeably dated. Even stuff like what characters are popular changes over time. And clothing can also go bad in storage - I sold some pyjamas that my eldest had outgrown on Vinted and was mortified when the buyer informed me that the elastic in the waistband had perished. They were probably ten years old. I hadn't thought to test the elastic when I was packaging them up.
I've decided to keep a couple of sentimental items in a memory box, the rest will go to a donation place so that people can use them while they still have life in them. Toys deserve to be played with, not stored in a box for decades. I have realised that if I miss something later and it was something that was mass-produced, it's highly likely that I can get hold of a second hand copy anyway. I've managed to get copies of favourite childhood books this way for my children to read, and the nostalgia is just as good even though it's not literally the same copy.
The space in your home has value as well, and storing things attracts dust, sometimes insects/spiders to nest in the dust and the dark crevices of stuff that never gets moved, and mould. It's also much much easier to keep our home tidier and it feels much nicer without piles of old stored stuff everywhere.
I keep things between DS2 and DS3 who only have 3 years between them. I don't keep clothing between DS1 and DS2. I keep books (because we live abroad and English books are hard to get) but I don't usually keep games and toys, unless the age span is so large that I think DS2 will be interested in them within a year or two.